Section 3.4 Introduction to the West African Monsoon

-10%

-20%

20%

10%

0

Wet period

Dry Period

The largest regional deficit of rainfall observed during the last centuryThe largest regional deficit of rainfall observed during the last century

Uncertainties about the futureUncertainties about the future

Climate Variability Impacts:

WaterAgricultureHealthDemographicsSecurity

Introduction to the West African Monsoon

Annual Cycle of Rainfall and associated Water Vapour Transport

Thorncroft et al (2011) QJRMS, 137, 129-147

The Coupled Monsoon System

Cold Tongue

SAL

ITCZ

Heat Low

Key features of the WAM Climate System during Boreal summer

AEJ

North-South Section along the Greenwich Meridian

θ

50oC

20oC

θ θe

90oC

60oC

θe

AEJ

Meridional Circulations

Shallow Meridional Circulation (SMC) over ocean, especially in Spring

θ

50oC

20oC

θ θe

90oC

60oC

θe

AEJ

Meridional Circulations

Data: GPCP (Global Precipitation Climatological Project). Resolution: pentad on a 2.5o grid. Averaged from 10oW to 10oE over 23 years (1979-2001).c.f. Gu and Adler (2004)

Annual Cycle of Mean Rainband

• Observations, reanalysis and operational analysis data including:

– pentad 2.5o GPCP

– Reynolds SST 1o, weekly and daily

– Reanalysis from the ECMWF: daily 2.5o ERA40

• The period of study is 1979-2001

Data

Relationship between SKT and surface meridional wind

SKT and rainfall

MSLP and VWND

Warming over the continent due to the surface solar heating.

Rapid cooling of the ocean surface south of the equator between April and June rapid rise in MSLP:

Acceleration of southerly winds across the equator.

c.f. Okumara and Xie (2004)

Relationship between rainfall and surface conditions

Relationship between rainfall and surface conditions

Equivalent potential temperature

• Peak rainfall always lies south of thetae peak• Gradient in thetae still important• Location of heat low important for poleward extent

Total Column Moisture Flux Convergence

Total Column Moisture Flux Convergence

Peak in moisture flux convergence linked to heat low shallow meridional circulation – acts to moisten the column and extend the rainfall polewards (c.f. Sultan and Janicot (2000,2003), Hagos and Cook (2008))

Total Column Moisture Flux Convergence

Peak in moisture flux convergence over ocean

Total Column Moisture Flux Convergence

Rapid shift and increase in moisture flux convergence towards coast between April and May

Total Column Moisture Flux Convergence

Rapid reduction in moisture flux convergence during June – linked to end of coastal rains

Total Column Moisture Flux Convergence

Rapid increase in moisture flux convergence beginning of July linked to Sahelian rainfall onset

Meridional Moisture Fluxes

Mid-levels (850-500mb)

Low-levels (sfc-850mb)

Impact of Heat Low SMC

Meridional Moisture Fluxes

Mid-levels (850-500mb)

Low-levels (sfc-850mb)

Equatorward moisture flux at mid-levels enhances moisture flux convergence in rainy zone : enhances rainfall there?

Polewards of this there is dry advection: inhibits rainfall there?

Meridional Moisture Fluxes

Mid-levels (850-500mb)

Low-levels (sfc-850mb)

Marked increase in cross-equatorial moisture fluxes during April-May

Linked to cold tongue development and coastal onset

Schematic evolution

SST

SMC

ITCZ1. Ocean phase (Feb-April): -Main rainband is broad with peak values just poleward

of the Equator (~1oN ). The rainfall is located mostly over the warmest water (>28oC) with little over the land.

-At the end of this period the cold tongue starts to develop, resulting in a broad region of SSTs close to the equator falling below 28oC.

- Does the heat low SMC impact the surface winds?

HL

2. Coastal phase (May-mid-June):-Cold tongue development associated with a rise in

equatorial surface pressure, and an acceleration of southerlies and associated moisture flux towards the coast.

-Marked moisture flux convergence, just equatorward of the land (~4oN) is associated with the highest rainfall of the annual cycle, and the first rainy season for coastal regions of West Africa.

c.f. Zheng, Eltahir and Emanuel (1999) Okumara and Xie (2004) Gu and Adler (2004)

Caniaux et al (2009)

- Peak rainfall is located over warmest water

Schematic evolution

SST

SMC

ITCZ

HL

3. Transitional Phase (End of June)- June represents a period where the environment

becomes less favorable for convection in the coastal region. This is consistent with coastal upwelling and a reduction of SSTs there.

- Intense coastal rainfall can only be transient?

- Why doesn’t it rain more in June?- Does this weakening promote the perception of a

“jump” often discussed in the literature?

Schematic evolution

SST

SMC

ITCZ

HL

4. Sahelian Phase (July-August): - Between June and July the peak in moisture flux

convergence reaches 10oN and increases rapidly consistent with the observed Sahelian rainfall onset.

- In July and August moisture flux divergence is present over the coastal region consistent with continued suppression of rainfall there.

c.f. Sultan and Janicot (2000.2003) Sijikumar et al (2006) Ramel et al (2006) Hagos and Cook (2007)

Schematic evolution

SST

SMC

ITCZ

HL

Wet bias in Spring?

Dry bias in Sahel in Summer

Dry bias in Spring?

ERA40 vs NCEP1rainfall

2005

2006

2007

11 20

21 3

23 29

Large variation in the coastal onset.

Earliest cold tongue development in Spring 2005 – earliest coastal onset.

Strongest HL in Spring 2007 during the oceanic regime possible role in delaying the coastal onset via subsidence

Concluding remarks on Annual Cycle

• At some level the coastal onset seems easier to understand than the Sahelian onset – with peak rainfall following the peak in SSTs

• What processes determine the nature and variability of the cold tongue (role of heat low, sub-surface ocean structure, Atlantic ocean variability, radiation)?

• Why is cold tongue development more rapid in the Atlantic than in the Pacific?

• Can climate models represent these coupled processes?

• Need more in situ observations in the tropical East Atlantic!

• Need more work on nature and causes of variability of coastal rains (next)

Annual Cycle of Synoptic Weather Systems

TD-filtered OLR (AEW-activity)Peaks in summer

We know little about the nature and causes of AEW-variability

Kelvin-filtered OLRPeaks in Spring

Key synoptic system for pre-coastal phase and possibly the coastal phase